Sensors for the determination of analytes in a sample solution are known in the prior art. Thus, in DE-A1-100 32 042, a system comprising a working electrode and a counter-electrode has been applied to a plastic carrier material. The working electrode is constructed of a number of layers, namely of at least one reaction layer and one protective layer. The protective layer is necessary because electrochemical signals in a sample solution are transferred to a sublimable mediator and thence to the working electrode. Without the protective layer, the mediator would sublime off and the sensor would be impaired in its function. The multilayer arrangement, however, makes it difficult to bring an exactly defined amount of sample or substrate solution into contact with the working electrode, which can lead to measuring inaccuracies. Moreover, the measurement requires a number of washing steps and prolongs the measuring time.
In WO-A1-99/58709, a testing device is described which consists of a carrier, an electrode system lying thereon and consisting of three electrodes of different dimensions and a number of net-like fabrics and covering layers deposited thereon. These layers and fabrics insulate the individual electrodes from one another. Moreover, they ensure that a measuring sample applied through an opening in the uppermost layer will be distributed through the flexible stack of layers around the electrodes more or less satisfactorily. However, the elastic stack will not absorb liquid samples applied in succession in exactly the same manner and disperse them between the individual layers in the same way every time. Therefore a constant amount of measuring sample cannot be caused to surround the electrodes in a defined volume range. As a result, measuring errors can occur and measurements show poor reproducibility. In other respects, the sandwich-like stack arrangement described is complicated to produce.
The testing device described in EP-B1-0 170 375 overcomes the disadvantages of a flexible construction. It discloses a measuring cell consisting of two plate-like hard plastic surfaces. These are arranged at a close distance from one another and enclose a particle-selective electrode system between them. The distance between the two surfaces from one another is set by ridges which run along the longitudinal sides of the surfaces. One of the two plastic surfaces is of larger construction than the other and forms a lip to which an analysis solution can be applied. Driven by capillary forces, this solution flows as far as the electrode system and at the same time causes air situated in front of it to be displaced in the downstream direction and thus to leave the testing device. Accordingly, analysis solution will flow into the testing device until the latter is completely filled with liquid. It is not possible to introduce further liquid, for example for washing purposes, into the interior of the testing device without beforehand laboriously having to remove the volume of solution already enclosed between the two plates by means of an absorbent fleece or fabric.
WO-A1-00/70327 also cannot completely overcome the aforementioned disadvantages. It discloses a testing apparatus for the quantitative electrochemical analysis of an analyte in a solid phase. It consists of a layer which contains electrodes and on which a supporting layer having a hole is disposed. The hole lies directly on top of the electrodes. A chromatographic test strip lies on the supporting layer. This is divided into three sections and consists of a sample collection pad adjoined by a flow surface, designated as a membrane, and an absorption pad. The sample collection pad contains a soluble substance specific to an analyte said substance carrying a label.